1. Field of the Invention
The present invention relates to a semiconductor device including at least one interdigitated transistor, the chip of which is mounted on the reverse side, i.e. by its active face, to a base used as a heat sink. The mounting can be done in such a way that the chip is self-encapsulated.
2. Description of the Prior Art
It is known that, in a semiconductor device, the active part is on a first face of a chip and that it has a thickness of the order of 1 to 10 micrometers while the substrate has a thickness of the order of 300 to 500 micrometers. However, during operation, heat is produced by the active part and has to cross the entire substrate to be discharged by the second face of the substrate, by means of a heat sink.
The problem exists for semiconductors on silicon substrates which, however, have fairly good qualities of heat conduction capacity, but it is aggravated for semiconductors on substrates made of III-V materials such as GaAs, which is a very poor conductor of heat, and it becomes a very irksome problem in the case of power semiconductors which dissipate a lot of heat.
The currently used approaches are difficult and costly to implement. Among many possibilities, we might cite:
the thinning of the substrate, by its rear face, to reduce the length of the heat transfer. However, this method embrittles the chip, especially if the circuit is a large-sized one; PA1 the implanting of a heat drain in the substrate, as shown in FIG. 1. If we consider a semiconductor reduced to an active layer 1 supported by a substrate 2, it is possible to etch a cavity in the substrate 2, up to the vicinity of the hottest region of the active layer 1, and to fill this cavity with a metal, which makes it a heat drain 3, in contact with a sink 4. But this technique is a delicate one. PA1 if the base 4 is itself a heat insulator, for example an alumina wafer in a hybrid circuit, it can be converted, in a manner similar to that described above, by being overlaid with metal regions 5 which improve the heat transfer coefficient, as shown in FIG. 1.
A further disadvantage of the different approaches is that they have a sequence of junctions between materials that are poor conductors of heat and materials that are good conductors of heat. This does not make for an optimum arrangement.
The invention proposes to dissipate the heat released during operation by means of a direct contact between the active layer of the semiconductor device and a heat sink, by brazing the chip "backwards" or in reverse to the sink.
However, in order to avoid short-circuiting the metallizations of the different electrodes of the semiconductor device, for example the source, gate and drain of a field-effect transistor, the electrodes of a same type, for example the source, which are at a same potential, are connected to one another by metal bridges, called "air" bridges, reinforced by an extra thickness in order to give them sufficient mechanical resistance. The invention can therefore be applied to an electronic circuit that includes at least one power transistor, which is itself interdigitated, including at least two fingers of a same electrode to enable a metal bridge to be set up between them.